High modulus crosslinked polyethylene with reduced residual free radical concentration prepared below the melt

ABSTRACT

The present invention provides an irradiated crosslinked polyethylene containing reduced free radicals, preferably containing substantially no residual free radical. Disclosed is a process of making irradiated crosslinked polyethylene by irradiating the polyethylene in contact with a sensitizing environment at an elevated temperature that is below the melting point, in order to reduce the concentration of residual free radicals to an undetectable level. A process of making irradiated crosslinked polyethylene composition having reduced free radical content, preferably containing substantially no residual free radicals, by mechanically deforming the polyethylene at a temperature that is below the melting point of the polyethylene, optionally in a sensitizing environment, is also disclosed herein.

This application is a continuation of U.S. Ser. No. 11/873,880, filedOct. 17, 2007, which is a continuation of U.S. Ser. No. 11/464,872,filed Aug. 16, 2006, which is a continuation of U.S. Ser. No.11/030,115, filed Jan. 7, 2005, now U.S. Pat. No. 7,166,650, which is acontinuation of U.S. Ser. No. 10/252,582, filed Sep. 24, 2002, now U.S.Pat. No. 6,852,772, which claims priority to U.S. Ser. No. 60/344,354,filed Jan. 4, 2002, the entireties of which are hereby incorporated byreference.

The present invention relates to irradiated crosslinked polyethylene(PE) compositions having reduced free radical content, preferablycontaining reduced or substantially no residual free radicals, andprocesses of making crosslinked polyethylene. The processes can comprisethe steps of irradiating the polyethylene while it is in contact with asensitizing environment at an elevated temperature that is below themelting point in order to reduce the concentration of residual freeradicals, preferably to an undetectable level. The invention alsorelates to processes of making crosslinked polyethylene having reducedfree radical content, preferably containing substantially no residualfree radicals, by mechanically deforming the irradiated PE either withor without contact with sensitizing environment during irradiation, at atemperature that is below the melting point of the polyethylene. Theseprocesses are complementary and can be used together or separately.

DESCRIPTION OF THE FIELD

Increased crosslink density in polyethylene is desired in bearingsurface applications for joint arthroplasty because it significantlyincreases the wear resistance of this material. The preferred method ofcrosslinking is by exposing the polyethylene to ionizing radiation.However, ionizing radiation, in addition to crosslinking, also willgenerate residual free radicals, which are the precursors ofoxidation-induced embrittlement. This is known to adversely affect invivo device performance. Therefore, it is desirable to reduce theconcentration of residual free radicals, preferably to undetectablelevels, following irradiation to avoid long-term oxidation.

In the past, in order to substantially reduce the concentration ofresidual free radicals in irradiated polyethylene, the polyethylene hasto be heated to above its melting temperature (for example, about 140°C.). Melting frees or eliminates the crystalline structure, where theresidual free radicals are believed to be trapped. This increase in thefree radical mobility facilitates the recombination reactions, throughwhich the residual free radical concentration can be markedly reduced.This technique, while effective at recombining the residual freeradicals, has been shown to decrease the final crystallinity of thematerial. This loss of crystallinity will reduce the modulus of thepolyethylene. Yet for high stress applications, such as unicompartmentalknee designs, thin polyethylene tibial knee inserts, low conformityarticulations, etc., high modulus is desired to minimize creep.

It is therefore desirable to reduce the residual free radicalconcentration without heating above the melting point in order to avoidsignificantly reducing the crystallinity of polyethylene, so as topermit insubstantial lowering, substantial maintenance, or an increasein the modulus.

SUMMARY OF THE INVENTION

An object of the invention to provide an improved irradiated crosslinkedpolyethylene having reduced concentration of free radicals, made by theprocess comprising irradiating the polyethylene at a temperature that isbelow the melting point of the polyethylene, optionally while it is incontact with a sensitizing environment, in order to reduce the contentof free radicals, preferably to an undetectable level, optionallythrough mechanical deformation.

In accordance with one aspect of the present invention, there isprovided an irradiated crosslinked polyethylene wherein crystallinity ofthe polyethylene is at least about 51% or more.

In accordance with another aspect of the present invention, there isprovided an irradiated crosslinked polyethylene, wherein the elasticmodulus of the polyethylene is higher or just slightly lower than, i.e.about equal to, that of the starting unirradiated polyethylene orirradiated polyethylene that has been subjected to melting.

According to the present invention, the polyethylene is a polyolefin andpreferably is selected from a group consisting of a low-densitypolyethylene, high-density polyethylene, linear low-densitypolyethylene, ultra-high molecular weight polyethylene (UHMWPE), ormixtures thereof.

In one aspect of the present invention, the polyethylene is contactedwith a sensitizing environment prior to irradiation. The sensitizingenvironment, for example, can be selected from the group consisting ofacetylene, chloro-trifluoro ethylene (CTFE), trichlorofluoroethylene,ethylene or the like, or a mixture thereof containing noble gases,preferably selected from a group consisting of nitrogen, argon, helium,neon, and any inert gas known in the art. The gas can be a mixture ofacetylene and nitrogen, wherein the mixture comprising about 5% byvolume acetylene and about 95% by volume nitrogen, for example.

In one aspect of the invention, the starting material of thepolyethylene can be in the form of a consolidated stock or the startingmaterial can be also in the form of a finished product.

In another aspect of the invention, there is provided an irradiatedcrosslinked polyethylene with reduced free radical concentration,preferably with no detectable residual free radicals (that is, thecontent of free radicals is below the current detection limit of 10¹⁴spins/gram), as characterized by an elastic modulus of about equal to orslightly higher than that of the starting unirradiated polyethylene orirradiated polyethylene that has been subject to melting. Yet in anotheraspect of the invention, there is provided a crosslinked polyethylenewith reduced residual free radical content that is characterized by animproved creep resistance when compared to that of the startingunirradiated polyethylene or irradiated polyethylene that has beensubjected to melting.

In accordance with one aspect of the invention there is provided amethod of making a crosslinked polyethylene comprising irradiating thepolyethylene at a temperature that is below the melting point of thepolyethylene while it is in contact with a sensitizing environment inorder to reduce the content of free radicals, preferably to anundetectable level.

In accordance with another aspect of the invention, there are providedmethods of treating crosslinked polyethylene, wherein crystallinity ofthe polyethylene is about equal to that of the starting unirradiatedpolyethylene, wherein crystallinity of the polyethylene is at leastabout 51% or more, wherein elastic modulus of the polyethylene is aboutequal to or higher than that of the starting unirradiated polyethyleneor irradiated polyethylene that has been subjected to melting.

There also is provided a method of making a crosslinked polyethylene,wherein the annealing temperature is below the melting point of thepolyethylene, wherein the annealing temperature is less than about 145°C., preferably less than about 140° C. and more preferably less thanabout 137° C.

Also provided herein, the material resulting from the present inventionis a polyethylene subjected to ionizing radiation with reduced freeradical concentration, preferably containing substantially no residualfree radicals, achieved through post-irradiation annealing at below themelting point at less than 145° C., preferably at less than 140° C. andmore preferably at less than 137° C., in the presence of a sensitizingenvironment.

In one aspect of the invention, there is provided a method of making acrosslinked polyethylene, wherein the polyethylene is contacted with asensitizing environment prior to irradiation.

In another aspect according to the present invention, there is provideda method of making a crosslinked polyethylene, wherein the sensitizingenvironment is acetylene, chloro-trifluoro ethylene (CTFE),trichlorofluoroethylene, ethylene gas, or mixtures of gases thereof,wherein the gas is a mixture of acetylene and nitrogen, wherein themixture comprises about 5% by volume acetylene and about 95% by volumenitrogen.

Yet in another aspect according to the present invention, there isprovided a method of making a crosslinked polyethylene, wherein thesensitizing environment is dienes with different number of carbons, ormixtures of liquids and/or gases thereof.

One aspect of the present invention is to provide a method of making acrosslinked polyethylene, wherein the irradiation is carried out usinggamma radiation or electron beam radiation, wherein the irradiation iscarried out at an elevated temperature that is below the meltingtemperature, wherein radiation dose level is between about 1 and about10,000 kGy.

In one aspect there is provided a method of making a crosslinkedpolyethylene, wherein the annealing in the presence of sensitizingenvironment is carried out at above an ambient atmospheric pressure ofat least about 1.0 atmosphere (atm) to increase the diffusion rate ofthe sensitizing molecules into polyethylene.

In another aspect there is provided a method, wherein the annealing inthe presence of sensitizing environment is carried with high frequencysonication to increase the diffusion rate of the sensitizing moleculesinto polyethylene.

Yet in another aspect there is provided a method of treating irradiatedcrosslinked polyethylene comprising steps of contacting the polyethylenewith a sensitizing environment; annealing at a temperature that is belowthe melting point of the polyethylene; and elevating the temperaturethat is below the melting point in presence of a sensitizing environmentin order to reduce the concentration of residual free radicals,preferably to an undetectable level.

Another aspect of the invention provides an improved irradiatedcrosslinked polyethylene composition having reduced free radicalconcentration, made by the process comprising irradiating at atemperature that is below the melting point of the polyethylene,optionally in a sensitizing environment; mechanically deforming thepolyethylene in order to reduce the concentration of residual freeradical and optionally annealing below the melting point of thepolyethylene, preferably at about 135° C., in order to reduce thethermal stresses.

In accordance with one aspect of the invention, mechanical deformationof the polyethylene is performed in presence of a sensitizingenvironment at an elevated temperature that is below the melting pointof the polyethylene, wherein the polyethylene has reduced free radicalcontent and preferably has no residual free radicals detectable byelectron spin resonance.

In accordance with another aspect of the invention the irradiation iscarried out in air or inert environment selected from a group consistingof nitrogen, argon, helium, neon, and any inert gas known in the art.

In accordance with still another aspect of the invention, the mechanicaldeformation is uniaxial, channel flow, uniaxial compression, biaxialcompression, oscillatory compression, tension, uniaxial tension, biaxialtension, ultra-sonic oscillation, bending, plane stress compression(channel die) or a combination of any of the above and performed at atemperature that is below the melting point of the polyethylene inpresence or absence of a sensitizing gas.

Yet in accordance with another aspect of the invention, mechanicaldeformation of the polyethylene is conducted at a temperature that isless than the melting point of the polyethylene and above roomtemperature, preferably between about 100° C. and about 137° C., morepreferably between about 120° C. and about 137° C., yet more preferablybetween about 130° C. and about 137° C., and most preferably at about135° C.

In one aspect, the annealing temperature of the irradiated crosslinkedpolyethylene is below the melting point of the polyethylene, preferablyless than about 145° C., more preferably less than about 140° C., andyet more preferably less than about 137° C.

Yet in another aspect, there is provided an irradiated crosslinkedpolyethylene, wherein elastic modulus of the polyethylene is about equalto or higher than that of the starting unirradiated polyethylene.

In accordance with the present invention, there is provided a method ofmaking an irradiated crosslinked polyethylene comprising irradiating ata temperature that is below the melting point of the polyethylene,optionally in a sensitizing environment; mechanically deforming thepolyethylene in order to reduce the concentration of residual freeradical and optionally annealing below the melting point of thepolyethylene, preferably at about 135° C., in order to reduce thethermal stresses.

In accordance with one aspect of the invention, there is provided amethod of mechanical deformation of polyethylene, optionally in presenceof a sensitizing environment, at an elevated temperature that is belowthe melting point of the polyethylene, preferably at about 135° C.,wherein the polyethylene has reduced free radical content and preferablyhas no residual free radical detectable by electron spin resonance.

In accordance with another aspect of the invention, there is provided amethod of deforming polyethylene, wherein the temperature is less thanthe melting point of the polyethylene and above room temperature,preferably between about 100° C. and about 137° C., more preferablybetween about 120° C. and about 137° C., yet more preferably betweenabout 130° C. and about 137° C., and most preferably at about 135° C.

Yet in another aspect of the present invention, there is provided amethod of treating irradiated crosslinked polyethylene composition inorder to reduce the residual free radicals comprising steps of:mechanically deforming the polyethylene; and annealing at a temperaturethat is below the melting point of the polyethylene in order to reducethe thermal stresses, wherein the mechanical deformation is performed(preferably at about 135° C.), optionally in presence of a sensitizingenvironment.

Still in another aspect of the invention, there is provided anirradiated crosslinked polyethylene composition made by the processcomprising steps of: irradiating at a temperature that is below themelting point of the polyethylene; mechanically deforming thepolyethylene below the melting point of the irradiated polyethylene inorder to reduce the concentration of residual free radicals; annealingat a temperature above the melting point; and cooling down to roomtemperature.

In another aspect, the invention provides a method of making anirradiated crosslinked polyethylene composition comprising steps of:mechanically deforming the polyethylene at a solid- or a molten-state;crystallizing/solidifying the polyethylene at the deformed state;irradiating the polyethylene below the melting point of thepolyethylene; and heating the irradiated polyethylene below the meltingpoint in order to reduce the concentration of residual free radicals andto recover the original shape or preserve shape memory.

Still in another aspect, the invention provides an irradiatedcrosslinked polyethylene composition made by the process comprisingsteps of: mechanically deforming the polyethylene at a solid- or amolten-state; crystallizing/solidifying the polyethylene at the deformedstate; irradiating the polyethylene below the melting point of thepolyethylene; and heating the irradiated polyethylene below the meltingpoint in order to reduce the concentration of residual free radicals andto recover the original shape or preserve shape memory.

Still in another aspect, the invention provides an irradiatedcrosslinked polyethylene with substantially reduced or no detectableresidual free radicals, wherein crystallinity of the polyethylene isabout 51% or greater.

These and other aspects of the present invention will become apparent tothe skilled person in view of the description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically the channel die set-up used in preparing someof the samples described in the Examples disclosed herein. The testsample A is first heated to a desired temperature along with the channeldie B. The channel die B is then placed in a compression molder and theheated sample A is placed and centered in the channel. The plunger C,which is also preferably heated to the same temperature, is placed inthe channel. The sample A is then compressed by pressing the plunger Cto the desired compression ratio. The flow direction (FD), walldirection (WD), and compression direction (CD) are as marked.

FIG. 2 shows schematically the oxidative aging or accelerated agingprocess and determination of residual free radicals thereafter. Aspecimen is prepared by cutting a 3 mm by 3 mm by 10 mm piece near thebody center with long axis of the specimen in the flow direction of thechannel die (see A). The specimen is then analyzed with electron spinresonance for residual free radicals. The remaining half of the testsample is further machined to obtain a cube with dimensions of 1 cm by 1cm by 1 cm. This cubic specimen (see B) is then subjected tothermo-oxidative aging or accelerated aging in air convection oven at80° C. for three weeks. This method of aging will induce oxidation inthe polyethylene if there are residual free radicals. At the completionof the aging, the cubic specimen is cut in half and microtomed to removea 200 micrometer thin section. The section is then analyzed using aBioRad UMA500 infra-red microscope as a function of depth away from theedge of the microtomed section as shown with arrow in the figure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes methods that allow reduction in theconcentration of residual free radicals in irradiated polyethylene,preferably to undetectable levels, without heating the material aboveits melting point. This method involves contacting the irradiatedpolyethylene with a sensitizing environment, and heating thepolyethylene to above a critical temperature that allows the freeradicals to react with the sensitizing environment, but is still belowthe melting point. It is likely that this critical temperaturecorresponds to the alpha transition of the polyethylene. The alphatransition of polyethylene is normally around 90-95° C.; however, in thepresence of a sensitizing environment that is soluble in polyethylene,the alpha transition may be depressed. The alpha transition is believedto induce motion in the crystalline phase, which is believed to increasethe diffusion of the sensitizing environment into this phase and/orrelease the trapped free radicals. The free radicals can now react withthe sensitizing gas and/or liquid, which are thought to act as a linkingagent between adjacent free radicals.

The material resulting from the present invention is a crosslinkedpolyethylene that has reduced residual free radicals, and preferably nodetectable free radicals, while not substantially compromising thecrystallinity and modulus.

According to the invention, the polyethylene is irradiated in order tocrosslink the polymer chains. In general, gamma irradiation gives a highpenetration depth but takes a longer time, resulting in the possibilityof some oxidation. In general, electron irradiation gives more limitedpenetration depths but takes a shorter time, and hence the possibilityof oxidation is reduced. The irradiation dose can be varied to controlthe degree of crosslinking and crystallinity in the final polyethyleneproduct. Preferably, a dose of greater than about 1 kGy is used, morepreferably a dose of greater than about 20 kGy is used. When electronirradiation is used, the energy of the electrons can be varied to changethe depth of penetration of the electrons, thereby controlling thedegree of penetration of crosslinking in the final product. Preferably,the energy is about 0.5 MeV to about 10 MeV, more preferably about 5 MeVto about 10 MeV. Such variability is particularly useful when theirradiated object is an article of varying thickness or depth, forexample, an articular cup for a medical prosthesis.

The invention also provides an improved irradiated crosslinkedpolyethylene, containing reduced free radical concentration andpreferably containing substantially no detectable free radicals, made bythe process comprising steps of contacting the irradiated polyethylenewith a sensitizing environment; annealing at a temperature that is belowthe melting point of the polyethylene; and elevating to a temperaturethat is below the melting point in presence of a sensitizing environmentin order to reduce the concentration of residual free radicals,preferably to an undetectable level.

The present invention provides methods of treating polyethylene, whereincrystallinity of the polyethylene is higher than that of the startingunirradiated polyethylene or irradiated polyethylene that has beenmelted, wherein crystallinity of the polyethylene is at least about 51%,wherein elastic modulus of the polyethylene is about the same as or ishigher than that of the starting unirradiated polyethylene.

The present invention also describes methods that allow reduction in theconcentration of residual free radical in irradiated polyethylene, evento undetectable levels, without heating the material above its meltingpoint. This method involves subjecting an irradiated sample to amechanical deformation that is below the melting point. The deformationtemperature could be as high as about 135° C. The deformation causesmotion in the crystalline lattice, which permits recombination of freeradicals previously trapped in the lattice through crosslinking withadjacent chains or formation of trans-vinylene unsaturations along theback-bone of the same chain. If the deformation is of sufficiently smallamplitude, plastic flow can be avoided. The percent crystallinity shouldnot be compromised as a result. Additionally, it is possible to performthe mechanical deformation on machined components without loss inmechanical tolerance. The material resulting from the present inventionis a crosslinked polyethylene that has reduced concentration ofresiduals free radical, and preferably substantially no detectable freeradicals, while not substantially compromising the crystallinity andmodulus.

The present invention further describes that the deformation can be oflarge magnitude, for example, a compression ratio of 2 in a channel die.The deformation can provide enough plastic deformation to mobilize theresidual free radicals that are trapped in the crystalline phase. Italso can induce orientation in the polymer that can provide anisotropicmechanical properties, which can be useful in implant fabrication. Ifnot desired, the polymer orientation can be removed with an additionalstep of annealing at an increased temperature below or above the meltingpoint.

According to another aspect of the invention, a high strain deformationcan be imposed on the irradiated component. In this fashion, freeradicals trapped in the crystalline domains likely can react with freeradicals in adjacent crystalline planes as the planes pass by each otherduring the deformation-induced flow. High frequency oscillation, such asultrasonic frequencies, can be used to cause motion in the crystallinelattice. This deformation can be performed at elevated temperatures thatis below the melting point of the polyethylene, and with or without thepresence of a sensitizing gas. The energy introduced by the ultrasoundyields crystalline plasticity without an increase in overalltemperature.

The present invention also provides methods of further annealingfollowing free radical elimination below melting point. According to theinvention, elimination of free radicals below the melt is achievedeither by the sensitizing gas methods and/or the mechanical deformationmethods. Further annealing of crosslinked polyethylene containingreduced or no detectable residual free radicals is done for variousreasons, for example:

1. Mechanical deformation, if large in magnitude (for example, acompression ratio of two during channel die deformation), will inducemolecular orientation, which may not be desirable for certainapplications, for example, acetabular liners. Accordingly, formechanical deformation:

-   -   a) Annealing below the melting point (for example, less than        about 137° C.) is utilized to reduce the amount of orientation        and also to reduce some of the thermal stresses that can persist        following the mechanical deformation at an elevated temperature        and cooling down. Following annealing, it is desirable to cool        down the polyethylene at slow enough cooling rate (for example,        at about 10° C./hour) so as to minimize thermal stresses. If        under a given circumstance, annealing below the melting point is        not sufficient to achieve reduction in orientation and/or        removal of thermal stresses, one can heat the polyethylene to        above its melting point.    -   b) Annealing above the melting point (for example, more than        about 137° C.) can be utilized to eliminate the crystalline        matter and allow the polymeric chains to relax to a low energy,        high entropy state. This relaxation will lead to the reduction        of orientation in the polymer and will substantially reduce        thermal stresses. Cooling down to room temperature is then        carried out at a slow enough cooling rate (for example, at about        10° C./hour) so as to minimize thermal stresses.

2. The contact before, during, and/or after irradiation with asensitizing environment to yield a polyethylene with no substantialreduction in its crystallinity when compared to the reduction incrystallinity that otherwise occurs following irradiation and subsequentmelting. The crystallinity of polyethylene contacted with a sensitizingenvironment and the crystallinity of radiation treated polyethylene isreduced by annealing the polymer above the melting point (for example,more than about 137° C.). Cooling down to room temperature is thencarried out at a slow enough cooling rate (for example, at about 10°C./hour) so as to minimize thermal stresses.

As described herein, it is demonstrated that mechanical deformation caneliminate residual free radicals in a radiation crosslinked UHMWPE. Theinvention also provides that one can first deform UHMWPE to a new shapeeither at solid- or at molten-state, for example, by compression.According to a process of the invention, mechanical deformation ofUHMWPE when conducted at a molten-state, the polymer is crystallizedunder load to maintain the new deformed shape. Following the deformationstep, the deformed UHMWPE sample is irradiated below the melting pointto crosslink, which generates residual free radicals. To eliminate thesefree radicals, the irradiated polymer specimen is heated to atemperature below the melting point of the deformed and irradiatedpolyethylene (for example, up to about 135° C.) to allow for the shapememory to partially recover the original shape. Generally, it isexpected to recover about 80-90% of the original shape. During thisrecovery, the crystals undergo motion, which can help the free radicalrecombination and elimination. The above process is termed as a‘reverse-IBMA’. The reverse-IBMA (reverse-irradiation below the melt andmechanical annealing) technology can be a suitable process in terms ofbringing the technology to large-scale production of UHMWPE-basedmedical devices.

These and other aspects of the present invention will become apparent tothe skilled person in view of the description set forth below.

A “sensitizing environment” refers to a mixture of gases and/or liquids(at room temperature) that contain sensitizing gaseous and/or liquidcomponent(s) that can react with residual free radicals to assist in therecombination of the residual free radicals. The gases maybe acetylene,chloro-trifluoro ethylene (CTFE), ethylene, or like. The gases or themixtures of gases thereof may contain noble gases such as nitrogen,argon, neon and like. Other gases such as, carbon dioxide or carbonmonoxide may also be present in the mixture. In applications where thesurface of a treated material is machined away during the devicemanufacture, the gas blend could also contain oxidizing gases such asoxygen. The sensitizing environment can be dienes with different numberof carbons, or mixtures of liquids and/or gases thereof. An example of asensitizing liquid component is octadiene or other dienes, which can bemixed with other sensitizing liquids and/or non-sensitizing liquids suchas a hexane or a heptane. A sensitizing environment can include asensitizing gas, such as acetylene, ethylene, or a similar gas ormixture of gases, or a sensitizing liquid, for example, a diene. Theenvironment is heated to a temperature ranging from room temperature toa temperature below the melting point of the material.

“Residual free radicals” refers to free radicals that are generated whena polymer is exposed to ionizing radiation such as gamma or e-beamirradiation. While some of the free radicals recombine with each otherto from crosslinks, some become trapped in crystalline domains. Thetrapped free radicals are also known as residual free radicals.

The phrase “substantially no detectable residual free radical” refers tono detectable free radical or no substantial residual free radical, asmeasured by electron spin resonance (ESR). The lowest level of freeradicals detectable with state-of-the-art instruments is about 10¹⁴spins/gram and thus the term “detectable” refers to a detection limit of10¹⁴ spins/gram by ESR.

The terms “about” or “approximately” in the context of numerical valuesand ranges refers to values or ranges that approximate or are close tothe recited values or ranges such that the invention can perform asintended, such as having a desired degree of crosslinking and/or adesired lack of free radicals, as is apparent to the skilled person fromthe teachings contained herein. This is due, at least in part, to thevarying properties of polymer compositions. Thus these terms encompassvalues beyond those resulting from systematic error.

The terms “alpha transition” refers to a transitional temperature and isnormally around 90-95° C.; however, in the presence of a sensitizingenvironment that dissolves in polyethylene, the alpha transition may bedepressed. The alpha transition is believed (An explanation of the“alpha transition temperature” can be found in Anelastic and DielectricEffects in Polymeric Solids, pages 141-143, by N. G. McCrum, B. E. Readand G. Williams; J. Wiley and Sons, N.Y., N.Y., published 1967) toinduce motion in the crystalline phase, which is hypothesized toincrease the diffusion of the sensitizing environment into this phaseand/or release the trapped free radicals.

The term “critical temperature” corresponds to the alpha transition ofthe polyethylene.

The term “below melting point” or “below the melt” refers to atemperature below the melting point of a polyethylene, for example,UHMWPE. The term “below melting point” or “below the melt” refers to atemperature less than 145° C., which may vary depending on the meltingtemperature of the polyethylene, for example, 145° C., 140° C. or 135°C., which again depends on the properties of the polyethylene beingtreated, for example, molecular weight averages and ranges, batchvariations, etc. The melting temperature is typically measured using adifferential scanning calorimeter (DSC) at a heating rate of 10° C. perminute. The peak melting temperature thus measured is referred to asmelting point and occurs, for example, at approximately 137° C. for somegrades of UHMWPE. It may be desirable to conduct a melting study on thestarting polyethylene material in order to determine the meltingtemperature and to decide upon an irradiation and annealing temperature.

The term “pressure” refers to an atmospheric pressure, above the ambientpressure, of at least about 1 atm for annealing in a sensitizingenvironment.

The term “annealing” refers to heating the polymer below its peakmelting point. Annealing time can be at least 1 minute to several weekslong. In one aspect the annealing time is about 4 hours to about 48hours, preferably 24 to 48 hours and more preferably about 24 hours. Theannealing time required to achieve a desired level of recovery followingmechanical deformation is usually longer at lower annealingtemperatures. “Annealing temperature” refers to the thermal conditionfor annealing in accordance with the invention.

The term “contacted” includes physical proximity with or touching suchthat the sensitizing agent can perform its intended function.Preferably, a polyethylene composition or pre-form is sufficientlycontacted such that it is soaked in the sensitizing agent, which ensuresthat the contact is sufficient. Soaking is defined as placing the samplein a specific environment for a sufficient period of time at anappropriate temperature. The environment include a sensitizing gas, suchas acetylene, ethylene, or a similar gas or mixture of gases, or asensitizing liquid, for example, a diene. The environment is heated to atemperature ranging from room temperature to a temperature below themelting point of the material. The contact period ranges from at leastabout 1 minute to several weeks and the duration depending on thetemperature of the environment. In one aspect the contact time period atroom temperature is about 24 hours to about 48 hours and preferablyabout 24 hours.

The term “Mechanical deformation” refers to a deformation taking placebelow the melting point of the material, essentially ‘cold-working’ thematerial. The deformation modes include uniaxial, channel flow, uniaxialcompression, biaxial compression, oscillatory compression, tension,uniaxial tension, biaxial tension, ultra-sonic oscillation, bending,plane stress compression (channel die) or a combination of any of theabove. The deformation could be static or dynamic. The dynamicdeformation can be a combination of the deformation modes in small orlarge amplitude oscillatory fashion. Ultrasonic frequencies can be used.All deformations can be performed in the presence of sensitizing gasesand/or at elevated temperatures.

The term “deformed state” refers to a state of the polyethylene materialfollowing a deformation process, such as a mechanical deformation, asdescribed herein, at solid or at melt. Following the deformationprocess, deformed polyethylene at a solid state or at melt is be allowedto solidify/crystallize while still maintains the deformed shape or thenewly acquired deformed state.

“IBMA” refers to irradiation below the melt and mechanical annealing.“IBMA” was formerly referred to as “CIMA” (Cold Irradiation andMechanically Annealed).

Sonication or ultrasonic at a frequency range between 10 and 100 kHz isused, with amplitudes on the order of 1-50 microns. The time ofsonication is dependent on the frequency and temperature of sonication.In one aspect, sonication or ultrasonic frequency ranged from about 1second to about one week, preferably about 1 hour to about 48 hours,more preferably about 5 hours to about 24 hours and yet more preferablyabout 12 hours.

By ultra-high molecular weight polyethylene (UHMWPE) is meant chains ofethylene that have molecular weights in excess of about 500,000 g/mol,preferably above about 1,000,000 g/mol, and more preferably above about2,000,000 g/mol. Often the molecular weights can reach about 8,000,000g/mol or more. By initial average molecular weight is meant the averagemolecular weight of the UHMWPE starting material, prior to anyirradiation. See U.S. Pat. No. 5,879,400; PCT/US99/16070, filed on Jul.16, 1999, WO 20015337, and PCT/US97/02220, filed Feb. 11, 1997, WO9729793, for properties of UHMWPE.

By “crystallinity” is meant the fraction of the polymer that iscrystalline. The crystallinity is calculated by knowing the weight ofthe sample (weight in grams), the heat absorbed by the sample in melting(E, in J/g) and the heat of melting of polyethylene crystals (ΔH=291J/g), and using the following equation:

% Crystallinity=E/w·ΔH

By tensile “elastic modulus” is meant the ratio of the nominal stress tocorresponding strain for strains as determined using the standard testASTM 638 M III and the like or their successors.

The term “conventional UHMWPE” refers to commercially availablepolyethylene of molecular weights greater than about 500,000.Preferably, the UHMWPE starting material has an average molecular weightof greater than about 2 million.

By “initial average molecular weight” is meant the average molecularweight of the UHMWPE starting material, prior to any irradiation.

The term “interface” in this invention is defined as the niche inmedical devices formed when an implant is in a configuration where thepolyethylene is in functional relation with another piece (such as ametallic or a polymeric component), which forms an interface between thepolymer and the metal or another polymeric material. For example,interfaces of polymer-polymer or polymer-metal in medical prosthesissuch as, orthopedic joints and bone replacement parts, e.g., hip, knee,elbow or ankle replacements. Medical implants containingfactory-assembled pieces that are in intimate contact with thepolyethylene form interfaces. In most cases, the interfaces are notaccessible to the ethylene oxide (EtO) gas or the gas plasma (GP) duringa gas sterilization process.

The piece forming an interface with polymeric material can be metallic.The metal piece in functional relation with polyethylene, according tothe present invention, can be made of a cobalt chrome alloy, stainlesssteel, titanium, titanium alloy or nickel cobalt alloy, for example.

The products and processes of this invention also apply to various typesof polymeric materials, for example, high-density-polyethylene,low-density-polyethylene, linear-low-density-polyethylene, UHMWPE, andpolypropylene.

The invention is further demonstrated by the following example, which donot limit the invention in any manner.

EXAMPLES Example 1 Channel Die Set-Up in Sample Preparation

Referring to FIG. 1, a test sample ‘A’ is first heated to a desiredtemperature along with the channel die B. The channel die ‘B’ is thenplaced in a compression molder and the heated sample A is placed andcentered in the channel. The plunger ‘C’, which also is preferablyheated to the same temperature, is placed in the channel. The sample ‘A’is then compressed by pressing the plunger ‘C’ to the desiredcompression ratio. The sample will have an elastic recovery afterremoval of load on the plunger. The compression ratio, λ (finalheight/initial height), of the test sample is measured after the channeldie deformation following the elastic recovery. The flow direction (FD),wall direction (WD), and compression direction (CD) are as marked inFIG. 1.

Example 2 Warm Irradiation with Sensitizing Gas Below the AlphaTransition

Test samples or a finished medical product of ultra-high molecularweight polyethylene (UHMWPE) are placed in a gas impermeable pouch (suchas polyethylene laminated aluminum foil), purged with a sensitizing gasand sealed with sensitizing gas substantially filling the package. Thepackage is then heated to a temperature between room temperature and 90°C. The package is then irradiated at the heated temperature using e-beamor gamma irradiation.

Example 3 Warm Irradiation with Sensitizing Gas Below the AlphaTransition with Subsequent Annealing in Sensitizing Gas

Test samples or a finished medical product of UHMWPE are placed in a gasimpermeable pouch (such as polyethylene laminated aluminum foil), purgedwith a sensitizing gas and sealed with sensitizing gas substantiallyfilling the package. The package is then heated to a temperature betweenroom temperature and 90° C. The package is then irradiated at the heatedtemperature using e-beam or gamma irradiation. The package is thenannealed at a temperature that is below the melting point ofpolyethylene.

Example 4 Warm Irradiation with Sensitizing Gas Above the AlphaTransition and Below the Melting Point

Test samples of UHMWPE are placed in a gas impermeable pouch (such aspolyethylene laminated aluminum foil), purged with a sensitizing gas andsealed with sensitizing gas substantially filling the package. Thepackage is then heated to a temperature between 90° C. and meltingtemperature (about 145° C.). The package is then irradiated at theheated temperature using e-beam or gamma irradiation.

Example 5 Warm Irradiation with Sensitizing Gas Above the AlphaTransition and Below the Melting Point with Subsequent Annealing inSensitizing Gas

Test samples of UHMWPE are placed in a gas impermeable pouch (such aspolyethylene laminated aluminum foil), purged with a sensitizing gas andsealed with sensitizing gas substantially filling the package. Thepackage is then heated to a temperature between 90° C. and meltingtemperature (about 145° C.). The package is then irradiated at theheated temperature using e-beam or gamma irradiation. The package isthen annealed at a temperature that is below the melting of pointpolyethylene.

Example 6 Post-Irradiation Contacting with a 5%/95% Acetylene/NitrogenGas Blend at an Elevated Temperature to Reduce the Concentration ofResidual Free Radicals

GUR 1050 ram-extruded UHMWPE bar stock (3.5″ diameter) was machined into4 cm thick cylinders. The cylinders were irradiated using anImpela-10/50 AECL 10 MeV electron beam accelerator (E-Beam Services,Cranberry N.J.) to a dose level of 100 kGy in air. The irradiatedcylinders were machined into 2 mm thick sections. Test samples wereprepared using sections with dimensions of 3×3×2 mm. Test samples wereplaced in polyethylene laminated aluminum foil pouches (three testsamples per pouch). Three of the pouches were purged with a 5%acetylene/95% nitrogen gas mixture (BOC Gas, Medford, Mass.) by pullingvacuum, then back-filling the pouch with the gas blend three times. Thepouches were sealed and left in slightly positive pressure of theacetylene/nitrogen gas blend. A fourth pouch was purged using the samemethod with 100% nitrogen gas and sealed with a slightly positivepressure of nitrogen gas inside the package.

Two of the acetylene/nitrogen-filled pouches and the nitrogen-filledpouch were then placed in a convection oven at 100° C. for 24 hours. Theother acetylene/nitrogen-filled pouch was kept at ambient temperaturefor 24 hours. The pouches were then opened, and the test samples wereanalyzed with electron spin resonance to determine the concentration ofresidual free radicals in the specimens. A set of three additional testsamples that were left in air at room temperature were also analyzedusing electron spin resonance. Results are shown in Table 1.

The results show that the irradiated test samples left in the 5%acetylene/95% nitrogen gas blend at room temperature for 24 hours hadsubstantial residual free radicals, as did the test samples stored inair at room temperature for 24 hours. The test samples left in the 100%nitrogen gas at 100° C. for 24 hours showed a slight decrease inresidual free radical concentration. The test samples left in 5%acetylene/95% nitrogen gas blend at 100° C. for 24 hours had nosubstantially detectable residual free radical. Therefore, the additionof 5% acetylene into nitrogen is sufficient to reduce the concentrationof the residual free radicals to undetectable levels following 100 kGyof electron beam irradiation.

TABLE 1 Concentration of residual free radicals measured in variousspecimens (n = 3 for all). E-Beam Post- Post-Irradiation Free radicalDose Irradiation Temperature Annealing concentration Test sample (kGy)Environment (° C.) time (hrs) [10¹⁵ spins/gram] As-Is followingirradiation 100 Air 25 Not applicable 8.67 ± 2.1 100% Nitrogen 100 100%nitrogen 100 24 3.99 ± 1.1 environment, 100° C. for 24 hours 5%/95%acetylene/nitrogen 100  5% acetylene 25 24 9.70 ± 0.2 gas environment,room temperature 5%/95% acetylene/nitrogen 100  5% acetylene 100 24 Notdetectable gas environment, 100° C. for 24 hours FIRST RUN 5%/95%acetylene/nitrogen 100  5% acetylene 100 24 Not detectable gasenvironment, 100° C. for 24 hours REPEAT RUN

Example 7 Irradiation of a Finished Polyethylene Medical Device in thePresence of a Sensitizing Gas at Room Temperature

A medical device is prepared from conventional UHMWPE and packaged in agas permeable material (such as Tyvek). It is then placed in gasimpermeable packaging (such as foil laminated packaging). This packageis then purged several times using a sensitizing atmosphere and wassealed in that atmosphere. The entire assembly is then irradiated usinggamma irradiation or e-beam to a dose level of 1 to 1000 kGy. Followingirradiation, the entire assembly is annealed. The annealing temperatureis selected such that the packaging remains intact and that at least onelevel of hermetic seal between the outside and the component is notbroken to maintain sterility of the medical device component. Thecomponent is then shipped for surgical use. If so desired, the remainingsensitizing gas is removed before shipping. The removal of thesensitizing gas is carried out by puncturing the package; or by removingthe outer foil pouch and shipping the component in the gas permeableinner package.

Example 8 Reduction of Residual Free Radicals in a Finished PolyethyleneMedical Device

A medical device made out of polyethylene with residual free radicals isplaced in a sensitizing atmosphere and annealed in the atmosphere thatis below the melting point of the polyethylene in order to reduce theconcentration of residual free radicals to at least substantiallyundetectable levels.

Example 9 Channel Die Deformation of Irradiated Polyethylene

Test samples of ultra-high molecular weight polyethylene are irradiatedat room temperature using e-beam or gamma radiation. The samples arethen placed in a channel die at 120° C., and are deformed in uniaxialcompression deformation by a factor of 2. The residual free radicalconcentration, as measured with electron spin resonance, are comparedwith samples held at 120° C. for the same amount of time.

Example 10 Channel Die Deformation of Irradiated Polyethylene Contactedwith a Sensitizing Environment

Test samples of ultra-high molecular weight polyethylene are irradiatedat room temperature using e-beam or gamma radiation. The samples arecontacted with a sensitizing gas, such as acetylene until saturated. Thesamples are then placed in a channel die at 120° C., and are deformed inuniaxial compression deformation by a factor of 2. The residual freeradical concentration, as measured with electron spin resonance, arecompared with samples held at 120° C. for the same amount of time.

Example 11 Warm Irradiation with Mechanical Annealing

Test samples of ultra-high molecular weight polyethylene are irradiatedat 120° C. adiabatically (that is, without significant heat loss to theenvironment) with electron beam radiation. The samples are then placedin a channel die at 120° C., and are deformed in uniaxial compressiondeformation by a factor of 2. The residual free radical concentration,as measured with electron spin resonance, is compared with samples heldat 120° C. for the same amount of time.

Example 12 Post-Irradiation Annealing in the Presence of 5%/95%Acetylene/Nitrogen Gas Mixed at an Elevated Temperature to Reduce theConcentration of Residual Free Radicals in a Large Polyethylene TestSample

GUR 1050 ram-extruded UHMWPE bar stock (3.5″ diameter) was machined into4 cm thick cylinders. The cylinders were irradiated using anImpela-10/50 AECL 10 MeV electron beam accelerator (E-Beam Services,Cranberry N.J.) to a dose level of 75 kGy in air. The irradiatedcylinders were machined into test samples with dimensions of about 2×2×2cm cubes. Two test samples were placed in two separate polyethylenelaminated aluminum foil pouches. One pouch was purged with a 5%acetylene/95% nitrogen gas mixture (BOC Gas, Medford, Mass.) by pullingvacuum, then back-filling the pouch with the gas blend. The pouch wassealed and left in slightly positive pressure of the acetylene/nitrogengas blend. The second pouch was purged using the same method with 100%nitrogen gas and sealed with a slightly positive pressure of nitrogengas inside the package.

Both pouches were then placed in a convection oven at 133° C. for 24hours. The pouches were then opened, and the test samples were furthermachined to prepare specimens for analysis with electron spin resonance.These specimens were prepared near the body center of the test samples.

The ESR analysis showed substantially no detectable free radicals in thespecimen prepared from the irradiated polyethylene that was annealedwhile in contact with 5%/95% acetylene/nitrogen gas mixture. Thespecimen prepared from the test sample that was annealed in 100%nitrogen showed a free radical signal, which was quantified to represent6×10¹⁴ spins/gram.

This example shows that the presence of even low concentrations of asensitizing gas such as 5% acetylene can reduce the concentration ofresidual free radicals in a large test sample with dimensions typical ofa polyethylene orthopedic implant without heating the said test sampleto above its melting point. This reduction in free radical concentrationis more than what is obtained by subjecting the same irradiatedpolyethylene to an identical thermal history in the presence of 100%nitrogen.

Example 13 Post-Irradiation Mechanical Deformation at an ElevatedTemperature to Reduce the Concentration of Residual Free Radicals

GUR 1050 compression molded UHMWPE bar stock was machined into cubes of4×4×4 cm dimensions. The cubes were irradiated using an gammairradiation to a dose level of 75 kGy in nitrogen. The irradiated cubeswere machined into test samples with dimensions of 2×2×1 cm. Two testsamples were placed in an air convection oven and heated to 135° C. inair, overnight (about 10 hours or more). One of the test samples wasthen placed in aluminum channel die, which was heated to 135° C., andpressed to a compression ratio, λ, of about two. The pressure was thenreleased and the sample was left to cool down to room temperature. Theother test sample was simply removed from the convection oven andallowed to cool down to room temperature with no mechanical deformation.

Both of these test samples were further machined. The test sample thatwas subjected to heating only was cut to remove a 5 mm long sliver(about 2×2 mm cross-section) from the body center. The other sample thatwas subjected to heating and channel die compression was cut to remove a5 mm long sliver (about 2×2 mm cross-section) from the body center. Thelong-axis of the sliver was parallel to the channel die flow direction.Both of these slivers were then analyzed with electron spin resonance.

The ESR analysis showed a free radical signal (which was quantified torepresent 2×10¹⁵ spins/gram) in the sliver that was prepared from thetest sample that was heated to 135° C. overnight. In contrast, thesliver prepared from the test sample that was heated to 135° C.overnight and mechanically deformed in the channel die (λ=2) at thattemperature showed no detectable residual free radicals. This exampleconfirms that mechanical deformation at an elevated temperature reducesthe concentration of residual free radicals.

Example 14 Determination of Crystallinity with Differential ScanningCalorimetry (DSC) Method

Differential scanning calorimetry (DSC) technique was used to measurethe crystallinity of the polyethylene test samples. The DSC specimenswere prepared from the body center of the polyethylene test sampleunless it is stated otherwise.

The DSC specimen was weighed with an AND GR202 balance to a resolutionof 0.01 milligrams and placed in an aluminum sample pan. The pan wascrimped with an aluminum cover and placed in the TA instruments Q-1000Differential Scanning Calorimeter. The specimen was first cooled down to0° C. and held at 0° C. for five minutes to reach thermal equilibrium.The specimen was then heated to 200° C. at a heating rate of 10° C./min.

The enthalpy of melting measured in terms of Joules/gram was thencalculated by integrating the DSC trace from 20° C. to 160° C. Thecrystallinity was determined by normalizing the enthalpy of melting bythe theoretical enthalpy of melting of 100% crystalline polyethylene(291 Joules/gram). As apparent to the skilled person, other appropriateintegration also can be employed in accordance with the teachings of thepresent invention.

The average crystallinity of three specimens obtained from near the bodycenter of the polyethylene test sample is recorded with a standarddeviation.

The Q1000 TA Instruments DSC is calibrated daily with indium standardfor temperature and enthalpy measurements.

Example 15 Crystallinity Measurements of Polyethylene FollowingIrradiation and Channel Die Deformation at an Elevated Temperature

GUR 1050 compression molded UHMWPE bar stock was machined into cubes of4×4×4 cm dimensions. The cubes were irradiated using gamma irradiationto a dose level of 75 kGy in nitrogen. The irradiated cubes weremachined into test samples with dimensions of 2×2×1 cm. One test sample(CIMA-12) was placed in an air convection oven and heated to 135° C. inair, overnight (10 hours). The test sample was then placed in analuminum channel die, which was heated to 135° C., and pressed to acompression ratio, λ, of about two. The pressure was then released andthe sample was left to cool down to room temperature.

The compressed test sample was further machined to prepare specimensfrom near the body center to be used to determine the crystallinity.Three such specimens obtained from near the body center were analyzedusing a TA instruments Differential Scanning Calorimeter at a heatingrate of 10° C./min and a temperature scan range of 0° C. to 200° C.

The enthalpy of melting (in terms of Joules/gram) was then calculated byintegrating the DSC trace from 20° C. to 160° C. The crystallinity wasdetermined by normalizing the enthalpy of melting by the theoreticalenthalpy of melting of 100% crystalline polyethylene (291 Joules/gram).

The average crystallinity of the three specimens obtained from near thebody center of the test sample was 58.9% with a standard deviation of0.7.

Example 16 Free Radical Concentration and Thermo-Oxidative Aging orAccelerated Aging Behavior of an Irradiated and Mechanically DeformedPolyethylene Sample

GUR 1050 compression molded UHMWPE bar stock was machined into cubes of4×4×4 cm dimensions. The cubes were irradiated using gamma irradiationto a dose level of 75 kGy in nitrogen. The irradiated cubes weremachined into test samples with dimensions of 2×2×1 cm. One test sample(CIMA-28) was placed in an air convection oven and heated to 135° C. inair for 4 hours. The test sample was then placed in an aluminum channeldie, which was heated to 135° C., and pressed to a compression ratio, λ,of about two. The pressure was then released and the sample was put backinto the air convection oven and heated for an additional 4 hours at135° C. to recover most of the plastic deformation.

A specimen was prepared by cutting a 3×3×10 mm piece near the bodycenter with long axis of the specimen in the flow direction of thechannel die (see A in FIG. 2). The specimen was analyzed with electronspin resonance and no free radicals were detected. The remaining half ofthe test sample was further machined to obtain a cube with dimensions of1×1×1 cm. This cubic specimen (see B in FIG. 2) was then subjected tothermo-oxidative aging or accelerated aging in air convection oven at80° C. for three weeks. This method of aging will induce oxidation inthe polyethylene if there are residual free radicals. At the completionof the aging, the cubic specimen was cut in half and microtomed toremove a 200 micrometer thin section. The section was then analyzedusing a BioRad UMA500 infra-red microscope as a function of depth awayfrom the edge of the microtomed section as shown in FIG. 2. Theinfra-red spectra collected with this method showed no detectablecarbonyl vibration throughout the microtomed section, indicating nodetectable oxidation. The crystallinity of the aged test sample was alsodetermined using three specimens cut form the said aged test sampleusing the DSC method described above in Example 14. The crystallinity ofthe three specimens averaged 59.2% with a standard deviation of 0.9 whenthe melting enthalpy was calculated by integrating the DSC trace from20° C. to 160° C.

The aging method provided additional support for the electron spinresonance in showing that irradiation followed by mechanical deformationat an elevated temperature results in a marked reduction in theconcentration of residual free radicals and an increase inthermo-oxidative stability of irradiated polyethylene.

Example 17 Annealing Following Free Radical Reduction Using Channel DieCompression at an Elevated Temperature

GUR 1050 UHMWPE bar stock was irradiated with gamma rays to 75 kGy innitrogen. The irradiated block was then machined to blocks withdimensions of 2×2×1 cm. Two of these blocks were placed in an airconvection oven at 133° C. for 4 hours. Both of these heated blocks werethen compressed in a channel die that was heated to 133° C. Thecompression ratio, λ=initial height/final height, was about two. Thedimensions of these blocks were measured and recorded after they werecooled down to room temperature (see Table 2).

One of the blocks (Block I in Table 2) was then annealed under no loadat 135° C. for 16 hours and cooled down to room temperature. Followingthis annealing cycle the dimensions of the block were measured again asshown in the Table 2. This observation shows that the plasticdeformation was markedly recovered by annealing below the melting point.

The other block (Block II in Table 2) was annealed under no load at 150°C. for 6 hours and cooled down to room temperature. Following thisannealing cycle the dimensions of the block were measured again as shownin Table 2. This observation shows that plastic deformation is almostfully recovered by annealing above the melting point.

TABLE 2 Annealing below and above melt using channel die compression atan elevated temperature. *Dimensions CD/FD/WD (mm) Following channel dieFollowing Sample Initial (mm) compression Annealing Block I 20 × 20 ×9.5 12 × 35 × 10 16.5 × 23.5 × 9.5 (Annealed below the melt) Block II 20× 20 × 9.5 10 × 40 × 10   20 × 20 × 9.5 (Annealed above the melt)*CD—Compression Direction; FD—Flow Direction; WD—Wall Direction

Example 18 Thermal Oxidative or Accelerated Aging Behavior of IrradiatedCross-Linked Polyethylenes that are Heated and Mechanically DeformedVersus an Irradiated Cross-Linked Heated Polyethylene

GUR 1050 UHMWPE bar stock was machined into blocks that were 9×9×4 cm.The blocks were gamma irradiated in a vacuum package to 100 kGy. Blockswere subsequently machined into the 19 mm cubes.

Four groups of cubes (n=2 for each temperature) were heated for one hourat 125° C., 128° C., 132° C., or 135° C., respectively. Subsequently,each heated cube was mechanically deformed between two flat aluminumplates held at room temperature to a compression ratio, λ, of 4.5. Thecompression displacement was held at this point for 5 minutes to allowfor stress relaxation to occur. The load required to hold thedisplacement constant at this point was monitored. By the end of thefive minutes the load had decreased and reached a steady state, at whichpoint the sample was removed from the press. All deformed cubes werethen annealed at 135° C. for 1 hour to partially recover deformation.Samples were then machined in half in the direction of compression toexpose an internal surface for accelerated aging.

Another four groups of cubes (n=2 for each group) were prepared to serveas thermal controls with no deformation history. These cubes weresubjected to the same thermal histories as those of the four groupsdescribed above. That is, the four groups were heated for one hour at125° C., 128° C., 132° C., or 135° C., respectively. The cubes were thenallowed to cool down to room temperature and annealed at 135° C. for 1hour. The thermal control samples were then machined in half in thedirection of compression to expose an internal surface for acceleratedaging.

The accelerated aging test specimens were placed in an air convectionoven at 80° C. and aged for 6 weeks. At the completion of aging, thesamples were cut in half and a 200 μm thin section was removed. The thinsection was scanned using a BioRad UMA 500 infrared microscope at 100micrometer intervals as a function of distance away from the exposedinternal free surface that was in contact with air during aging. Thescans were used to find the location of the maximum carbonyl vibration.The infrared spectrum collected at this maximum carbonyl location wasused to assign an oxidation index to that aged cube. The oxidation inindex was calculated by normalizing the area under the carbonylvibration to that under the 1370 cm⁻¹ vibration. The higher theoxidation in the sample, the stronger is the carbonyl vibration and as aresult higher is the oxidation index.

The oxidation indexes of the four groups of deformed samples were lessthan 0.03. In contrast, the thermal control groups showed oxidationindexes of 1.3, 1.2, 1.2, and 1.3 for the pre-heat temperatures of 125°C., 128° C., 132° C., or 135° C., respectively.

Based on above results, it is concluded that heating alone (below themelting point) does not improve the oxidation resistance of irradiatedand cross-linked polyethylene to the same extent as heating andsubsequent deformation do.

It is to be understood that the description, specific examples and data,while indicating exemplary aspects, are given by way of illustration andare not intended to limit the present invention. Various changes andmodifications within the present invention will become apparent to theskilled artisan from the discussion, disclosure and data containedherein, and thus are considered part of the invention.

1. A method of making a medical prosthesis comprising irradiated crosslinked polyethylene composition, wherein the method comprises the steps of: a) irradiating polyethylene at a temperature that is below the melting point of the polyethylene in order to form free radicals that crosslink by recombination; and b) mechanically deforming the polyethylene from step a) at a temperature that is above room temperature and below the melting point of the irradiated polyethylene in order to mobilize residual free radicals that are trapped to allow residual free radicals to recombine and form additional crosslinks, thereby providing a medical prosthesis having irradiated crosslinked polyethylene composition, wherein molecules of the irradiated crosslinked polyethylene composition are orientated through the mechanical deforming. 2-62. (canceled)
 63. The method of claim 1, wherein the mechanical deformation mode is selected from the group consisting of channel flow, uniaxial compression, biaxial compression, oscillatory compression, uniaxial tension, biaxial tension, ultra-sonic oscillation, bending, plane stress compression (channel die), and combinations thereof.
 64. The method of claim 1, wherein the polyethylene is mechanically deformed by bending.
 65. The method of claim 1, wherein the medical prosthesis is an orthopedic joint or a bone replacement part selected from the group consisting of a hip, a knee, an elbow, or an ankle replacement part.
 66. The method of claim 1, wherein the mechanical deformation of the polyethylene is carried out on a machined components.
 67. A method of achieving molecular orientation in an irradiated crosslinked polyethylene composition, wherein the method comprises the steps of: a) irradiating polyethylene at a temperature that is above the melting point of the polyethylene in order to form free radicals that crosslink by recombination, thereby forming a crosslinked polyethylene; b) mechanically deforming the crosslinked polyethylene from step a) at a molten-state in order to mobilize residual free radicals that are trapped to allow residual free radicals to recombine and form additional crosslinks, thereby providing an irradiated crosslinked polyethylene composition, wherein molecules of the irradiated crosslinked polyethylene composition are orientated through the mechanical deforming.
 68. The method of claim 67, wherein the mechanical deformation mode is selected from the group consisting of channel flow, uniaxial compression, biaxial compression, oscillatory compression, uniaxial tension, biaxial tension, ultra-sonic oscillation, bending, plane stress compression (channel die), and combinations thereof.
 69. The method of claim 67, wherein the polyethylene is mechanically deformed by bending.
 70. The method of claim 67, wherein the polyethylene is irradiated at a dose level between about 1 and about 1,000 kGy.
 71. The method of claim 1, wherein the mechanical deformation of the polyethylene is carried out on a machined components.
 72. A method of imparting molecular orientation in an irradiated crosslinked polyethylene composition, wherein the method comprises the steps of: a) mechanically deforming polyethylene at a solid- or a molten-state, thereby providing a polyethylene composition having molecular orientation; and b) irradiating the polyethylene of step a) at the deformed state at a temperature that is below the melting point of polyethylene in order to form free radicals that crosslink by recombination, thereby providing an irradiated crosslinked polyethylene composition, wherein molecules of the irradiated crosslinked polyethylene composition are orientated through the mechanical deforming.
 73. The method of claim 72, wherein the polyethylene is ultra-high molecular weight polyethylene (UHMWPE).
 74. The method of claim 72, wherein the mechanical deformation mode is selected from the group consisting of channel flow, uniaxial compression, biaxial compression, oscillatory compression, uniaxial tension, biaxial tension, ultra-sonic oscillation, bending, plane stress compression (channel die), and combinations thereof.
 75. The method of claim 72, wherein the polyethylene is mechanically deformed by bending.
 76. The method of claim 72, wherein polyethylene composition is a medical implant.
 77. A method of fabricating a medical implant comprising a crosslinked polymeric material having molecular orientation, wherein the method comprises the steps of: a) mechanically deforming the medical implant at a temperature that is below the melting temperature of the polymeric material; and b) irradiating the medical implant of step a) at the deformed state at a temperature that is below the melting point of the polymeric material in order to form free radicals that crosslink by recombination, thereby providing a medical implant comprising the crosslinked polymeric material, wherein molecules of the crosslinked polymeric material are orientated through the mechanical deforming.
 78. The method of claim 77, wherein the mechanical deformation mode is selected from the group consisting of channel flow, uniaxial compression, biaxial compression, oscillatory compression, uniaxial tension, biaxial tension, ultra-sonic oscillation, bending, plane stress compression (channel die), and combinations thereof.
 79. A medical implant made according to the method of claim
 77. 80. The medical implant according to claim 79 is an orthopedic joint or a bone replacement part selected from the group consisting of a hip, a knee, an elbow, or an ankle replacement part.
 81. The medical implant according to claim 79 is a thin polyethylene tibial knee insert, low conformity articulations, or acetabular liner.
 82. A method of fabricating a medical implant comprising a crosslinked polymeric material having molecular orientation, wherein the method comprises the steps of: a) irradiating the medical implant at a temperature that is below the melting point of the polymeric material in order to form free radicals that crosslink by recombination, thereby forming a medical implant having crosslinked polymeric material; and b) mechanically deforming the medical implant having crosslinked polymeric material from step a) in order to mobilize residual free radicals that are trapped to allow residual free radicals to recombine and form additional crosslinks, thereby providing a medical implant comprising the crosslinked polymeric material, wherein molecules of the crosslinked polymeric material are orientated through the mechanical deforming.
 83. The method of claim 82, wherein the mechanical deformation mode is selected from the group consisting of channel flow, uniaxial compression, biaxial compression, oscillatory compression, uniaxial tension, biaxial tension, ultra-sonic oscillation, bending, plane stress compression (channel die), and combinations thereof.
 84. A medical implant made according to the method of claim
 82. 85. The medical implant according to claim 84 is an orthopedic joint or a bone replacement part selected from the group consisting of a hip, a knee, an elbow, or an ankle replacement part.
 86. The medical implant according to claim 84 is a thin polyethylene tibial knee insert, low conformity articulations, or acetabular liner. 